Systems and Methods for Comparative Interferogram Spectrometry

ABSTRACT

A method for determining a background noise level includes receiving interferogram data; determining at least one measure of interferogram quality; accumulating said received interferogram data; and generating a background noise level based on said interferogram data and at least one measure of interferogram quality.

RELATED APPLICATIONS

The present application is a continuation and claims the priority under35 U.S.C. §120 of U.S. patent application Ser. No. 11/542,465, filedOct. 2, 2006, entitled “Systems and Methods for ComparativeInterferogram Spectrometry,” which application is currently allowed andis incorporated herein by reference in its entirety.

BACKGROUND

Remote chemical identification is an increasingly important tool forenvironmental agencies, military groups, manufacturers, and otherorganizations. Many chemical plants, manufacturing plants, waterutilities, military bases, and other facilities use and store hazardouschemicals. In the event that chemicals are released, it is desirable toidentify the chemicals and assess the threat from a distance. Ideally,potentially hazardous chemicals should be identified without exposinghuman investigators to the chemicals. Remote identification, known asstand-off detection, can be effective over a large area and may be usedin a variety of ambient conditions.

Conventional stand-off chemical identification systems traditionallyemploy Fourier transform infrared (FTIR) spectroscopy to identifychemicals in the atmosphere. FTIR spectroscopy, however, is sensitive tonoise and performs poorly in changing background conditions. Thecomputational demands of Fourier transform analysis also slow theanalysis process. Furthermore, the Fourier transform removes valuableinformation from interferogram data and limits the precision of theanalysis. Additionally, many FTIR systems require blackbody radiationsources and other expensive hardware for calibration.

SUMMARY

A system for comparative interferogram spectrometry includes aninterferometer configured to generate interferograms from incidentradiation from a target region, an interferogram database containingstored interferograms, and a processing subsystem configured to receivethe generated interferograms and compare the received interferograms tothe stored interferograms.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of theprinciples described in this specification and are a part of thespecification. The illustrated embodiments are merely examples and donot limit the scope of the principles described herein.

FIG. 1 is a diagram illustrating a typical use for a comparativeinterferogram spectrometry system, according to one exemplaryembodiment.

FIG. 2 is a block diagram illustrating components of a comparativeinterferogram spectrometry system, according to one exemplaryembodiment.

FIG. 3 is a schematic illustrating an arrangement of interferometeroptics, according to one exemplary embodiment.

FIG. 4 illustrates an interferogram generated by a comparativeinterferogram spectrometry system, according to one exemplaryembodiment.

FIG. 5 illustrates another interferogram generated by a comparativeinterferogram spectrometry system, according to one exemplaryembodiment.

FIGS. 6A through 6E illustrate various elements of background processingin a comparative interferogram spectrometry system, according to variousexemplary embodiments.

FIG. 7 is a flowchart illustrating a method for determining a backgroundnoise level, according to one exemplary embodiment.

FIG. 8 is a flowchart illustrating a method for identifying chemicalsusing interferogram data, according to one exemplary embodiment.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

Various exemplary methods and systems for comparative interferogramspectrometry (CIS) are described herein. CIS systems analyzeinterferogram data rather than spectral data. Consequently, CIS systemsbenefit from information that spectral approaches normally eliminatewith a Fourier transform. Using comparative information theory and theinterferogram partitioning principle, CIS can identify chemicalsignatures from a distance without blackbody calibration or artificiallight sources. As will be explained in greater detail below, comparativeinformation theory (CIT) enables high sensitivity comparisons ofinterferograms using the ratio of interferograms. The sensitivity of aCIS system is further enhanced by calculating CIT ratios from compositeinterferograms which incorporate information from multiple measurements.Application of the interferogram partitioning principle (IPP) enables aCIS system to exclude noisy sections of an interferogram from analysisthat would be inextricable from spectral analysis. The excellentsensitivity of CIS can also be used to characterize the performance andstability of measurement instruments.

As used herein and in the appended claims, the term “target region” isdefined to include any location, area, or region of measurement withinthe optical path of the interferometer which contributes to the lightreceived and consequently the interferogram generated.

As used herein and in the appended claims, the term “compositeinterferogram” is defined to include any interferogram that containsinformation describing, derived from, or representing at least twointerferograms. Each component interferogram contributing to a compositeinterferogram need not contribute an equal amount or type ofinformation. Additionally, a composite interferogram may contain only aportion of the information contained in its component interferograms.Furthermore, the initial information contributed to form a compositeinterferogram need not be separable from the composite interferogram.Composite interferograms may be created by processes including, but notlimited to, summation, integration, piecewise construction, ratio ofinterferograms, and additional physical and mathematical operations.

As used herein and in the appended claims, the term “scaling” is definedto include the alteration of one or more characteristics of a data set.For a two dimensional interferogram, for example, scaling may include,but is not limited to, expansion or compression along the horizontalaxis, vertical axis, both axes, or a portion or either or both axes.Scaling may include normalizing a data set so that the maximum orminimum intensity is the same across the multiple measurements. Scalingmay also adjust one or more interferograms to achieve a certain value orrange of values of width, area, shape, ratio, and other characteristics.Scaling may adjust a data set so that relative intensities or othercharacteristics are preserved even though other properties are altered.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present systems and methods. It will be apparent,however, to one skilled in the art that the present systems and methodsmay be practiced without these specific details. Reference in thespecification to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least one embodiment. Theappearance of the phrase “in one embodiment” in various places in thespecification are not necessarily all referring to the same embodiment.

As illustrated in FIG. 1, a CIS system may be used to remotely identifychemicals (100) in the atmosphere. A CIS system (110) may detect wavesemitted, reflected, transmitted, or otherwise propagated to the CISsystem (110) from released chemicals (100) and a background (130).According to one embodiment, the CIS system (110) may be configured forstationary use or may be installed in a vehicle or other mobileplatform.

The target region (120) indicated by dashed lines indicates that wavesmay be received from the chemicals (100) to be identified and from thebackground (130). Additionally, the size, shape and resolution of thetarget region (120) may be altered to suit measurement conditions.According to one embodiment, the CIS system (110) of FIG. 1 may beconfigured to identify the chemicals (100) in the target region (120) bydetecting waves entering the system (110) without emitting anyradiation.

The block diagram of FIG. 2 introduces the components of an exemplaryCIS system. An interferometer (200) receives incident waves andgenerates interferograms from the waves received. According to oneembodiment, the interferometer (200) may be configured to receiveinfrared (IR) light or other incident waves. A processing subsystem(210) receives the interferogram data and may compensate for backgroundnoise, instrument error, and other factors. The processing subsystem(210) then compares the received interferograms with referenceinterferograms recorded in an interferogram database (220) to identifythe chemicals detected. The results may then be presented with one ormore output devices (230).

The interferometer (200) includes optics (202) to generateinterferograms from incident light. According to one embodiment, theoptics (202) may be configured to generate interferograms from infrared(IR) light. According to another embodiment, the optics (202) may beconfigured to generate interferograms from other electromagneticradiation including, but not limited to, terahertz waves, near infraredlight, visible light, ultraviolet light, and x-rays. Optics may include,but are not limited to, lenses, mirrors, prisms, windows, shutters,filters, beam splitters, collimators, and other optical instruments.Lenses and other optics may include quartz, calcium fluoride (CaF₂),germanium, silicon, polyethylene Fresnel lenses, or other materials, andmirrors may be made of aluminum, gold, or other reflective materials.According to one embodiment, the optics (202) may be selected andconfigured for best performance with a specific range of wavelengths ofradiation, such as infrared light between 750 nm and 1 mm, 3 μm and 8μm, or other wavelength range.

An exemplary configuration of interferometer optics (202) is illustratedin FIG. 3. According to the optical configuration (300) of FIG. 3,incident waves (360) enter the interferometer (200) and are divided by abeam splitter (330). Essentially half of the waves (362) may bereflected while the remaining waves (364) are transmitted through thebeam splitter (330). Initially reflected waves (362) reflect again froma fixed mirror (310) and are then transmitted to the detector (340) viaa first optical path. The initially transmitted waves (364) follow asecond path and are reflected by a moving mirror (320) to the detector(340) along a second optical path. As the moving mirror (320) moves awayfrom the beam splitter (330), the second optical path length (364)increases. The variation in distance between the two paths causesinterference patterns to be generated at the detector (340), similar tothose shown in FIGS. 4 and 5.

A variety of additional optical configurations (300) are compatible withthe interferometer (200, FIG. 2) in the CIS system (110, FIG. 2). Thearrangement of optics (300) in FIG. 3 is intended to be purelyillustrative and not restrictive in any way.

Continuing with FIG. 2, interferogram data generated by the optics (202)is detected by a sensor (204). Sensors (204) may include, but are notlimited to, photoconductive detectors, photovoltaic detectors,pyroelectric detectors, and photoacoustic detectors, photodiodes, andphototransistors. A sensor (204) may include variety of materials, suchas deuterated triglycine sulfate (DTGS), deuterated lanthanum triglycinesulfate (DLATGS), ceramic lead zirconate titanate (PZT), indiumantimonide (InSb), indium arsenide (InAs), indium gallium arsenide(InGaAs), lead sulfide (PbS), lead selenide (PbSe), lithium tantalate(LiTaO₃), silicon (Si), mercury zinc telluride (HgZnTe, MZT), mercurycadmium telluride (HgCdTe, MCT), and other IR-sensitive substances. Somedetectors, such as those containing HgCdTe, may be cooled using liquidnitrogen or other coolant systems. Two or more IR sensitive materialsmay be combined in a detector for improved sensitivity over a widerrange of wavelengths.

A variety of detectors may be configured for use with an interferometer(200). For example, one sensor (204) may exhibit excellent signal tonoise characteristics but require a large cooling system, while asmaller, less sensitive sensor (204) may still be adequate for someapplications. As with the optics (202), the sensor (204) may be selectedor tuned for a range of wavelengths suitable for the chemicals to bedetected.

A variety of sensor (204) configurations may be used to maximize theeffective range, precision, signal to noise ratio, detectable area, andother characteristics of the CIS system (110). According to oneembodiment, one or more sensors (204) may be configured to scan an area.At various measurement points, the interferometer (200) may employcontinuous scan or step-scan data acquisition methods. According toanother embodiment, multiple sensors (204) may be arranged in a linearor two-dimensional array to improve resolution and decrease dataacquisition time.

Processing circuitry (206), including amplifiers, filters, and otherdevices, may be used to improve the quality of signals generated by thesensor elements (204). Amplifiers and other circuit elements may bedesigned to take into account temperature changes of the interferometer(200), the electrical properties of the sensors (204), and othercharacteristics of the CIS system (110).

Interferogram data generated by the interferometer (200) may betransmitted to the processing subsystem (210) through a variety oftransmission media. Transmission media may include, but are not limitedto, coaxial cables, fiber optics, and wires, including the wires thatcomprise a system bus coupled to a processor of a computer. Transmissionmedia may also include or convey acoustic waves, light waves, andelectromagnetic emissions, such as those generated during radiofrequency (RF) and infrared (IR) data communications.

As shown in FIG. 2, the processing subsystem (210) may include one ormore processors, such as a processor (212) configured to control theoperations of the processing subsystem (210). The processor (212) may beconfigured to execute commands to perform tasks such as receive andtransmit data, generate output data, compare an interferogram to recordsin an interferogram database (220), and other functions which will bedescribed in greater detail herein.

A signal processing unit (216) may assist one or more processors (212)in the processing of interferogram data. According to one embodiment,the signal processing system (210) may apply one or more filters, scalereceived data, calculate background levels, determine validity ofreceived data, compare interferogram data, and perform other processingof interferogram data.

The processing subsystem (210) may also include memory (214). Memory(214) may include, but is not limited to, FLASH memory, random accessmemory (RAM), dynamic RAM (DRAM), optical or magnetic disks, and othervolatile or non-volatile memory. Memory (214) may store applications,such as analysis software, as well as received interferogram data,analysis results, and other data or commands.

According to one embodiment, an interferogram database (220) may belinked to the processing subsystem (210) through one or moretransmission media. Additionally or alternately, an interferogramdatabase (220) may be integrated into a processing subsystem (210). Forexample, the interferogram database (220) may include lists, objects,trees, graphs, tables, arrays, records, or other data structures inmemory (214). An interferogram database (220) may be implemented inmemory (214) associated with one or more processors (212) and/or signalprocessing units (216) to allow interferogram data to be stored orretrieved.

An interferogram database (220) may store interferograms that may becompared to interferograms generated by the interferometer (200). Theinterferogram database (220) may also store distinguishing interferogramfeatures or fringe ranges that may facilitate identification ofchemicals. Since information about the IR frequencies received isincluded in multiple fringes, the interferograms stored in theinterferogram database contain more points of comparison to receiveddata than spectral plots. Since detection, identification, and all otherprocessing may be completed in the interferogram domain, no spectraldata is necessary.

Although spectral data is not necessary, some embodiments of theinterferogram database (220) or processing subsystem (110) may includespectral data for analysis or display. According to one embodiment,spectral data may provide a useful visual representation of the analysisresults. A spectral representation of the interferogram data analyzedmay be provided for a user to view even though analysis is performed inthe interferogram domain. According to another embodiment, chemicalanalysis may be performed in the spectral domain and the interferogramdomain simultaneously. In some embodiments, the use of both spectral andinterferogram approaches may improve the accuracy, speed, or otheraspects of interferogram processing.

According to another embodiment, the interferograms stored in theinterferogram database (220) may be generated in ambient conditions.Interferograms generated in controlled laboratory settings may not beadequately comparable to interferograms generated in ambient conditions.Reference interferogram data may also represent the accumulation orintegration of multiple interferograms as a composite interferogram.Reference interferograms may be generated with a single chemical ormultiple chemicals of interest, and may be stored with or withoutbackground data present.

According to one embodiment, the interferogram database (220) mayinclude interferograms and/or composite interferograms matchedspecifically to the interferometer configuration used to generate thedata interferograms. According to another embodiment, a referencelibrary of interferograms may be generated using the specific hardwareelements of the interferometer to be used for remote chemicalidentification. Since measurements may vary significantly betweenidentically configured interferometers, reference libraries generatedfor individual instruments may improve analysis sensitivity.

One or more output devices (230) may be attached through appropriatetransmission media to the processing subsystem (210) to present theresults of the chemical identification process or other data generated.Spectral data, interferogram data, visual data, system messages, andother information may also be presented. According to one embodiment, anoutput device may include a screen such as a computer monitor, liquidcrystal display (LCD), a cathode ray tube (CRT), or other visualdisplay. Additionally, a printer, speaker, or other output device may beattached to present information to a user of the CIS system (110).Specifically, with a speaker, chemical identification data or warningscan be presented audibly.

An exemplary interferogram is illustrated in FIG. 4, according to oneembodiment. The interferogram (400) represents the constructive anddestructive interference of many incident waves. According to theembodiment of FIG. 4, the interferogram represents the combinedintensity of incident IR waves received on the vertical axis versus timeon the horizontal axis.

While each IR frequency is represented at a single point on a spectralplot, information about each IR frequency present will be included inmultiple fringes of an interferogram (400). For each feature of interestin the spectral domain, there are many corresponding fringes in theinterferogram domain that may be used to identify a chemical.

As illustrated in FIG. 4, a valid interferogram (400) may be essentiallysymmetrical about a central fringe (410). Corresponding interferogramsections (420, 430) may be compared to determine if symmetry isacceptable and thus if an interferogram (400) contains useful data.Interferogram features that exist in only one half of an interferogramare generally the result of noise and not actual chemicals in themonitored area.

An interferogram (500) of questionable validity is shown in FIG. 5. Asshown in FIG. 5, the right side (510) of the interferogram (500) doesnot correspond with the left side of the interferogram (500). Asymmetryin an interferogram (500) may be caused by noise, rapidly changingbackgrounds, instrument error, quickly changing temperatures, and otherconditions. According to one embodiment, the interferometer (200, FIG.2) or processing subsystem (210, FIG. 2) may identify and quantifyinstrument error. In such a case, the interferogram (500) can bevalidated.

In many interferometers (100), instrument error may cause systematicshifting, stretching, asymmetries, and other distortions in measuredinterferograms. According to one embodiment, the processing subsystem(210) may compensate for small, consistent distortions in interferogramsreceived. Additionally, the processing subsystem (210) may calculate theseverity of asymmetries in received interferograms and compare thecalculated asymmetry level to an asymmetry threshold. Interferogramswith asymmetry exceeding a threshold level may be ignored as invaliddata. To determine the degree of asymmetry present in an interferogram,the processing subsystem (210, FIG. 2) may calculate the ratio betweenthe accumulated intensity of each half of an interferogram, the ratio ofintensities of corresponding sections, or the ratio of area or intensityof individual corresponding fringes. The placement of interferencefringes, maximum and minimum intensities, and other interferogramcharacteristics may also be compared to determine asymmetry andcompensate for instrument error.

Returning to FIG. 4, signal strength and interferogram intensitygenerally decrease as distance from the central fringe (410) increases.Noise contributions become an increasingly greater proportion of theinterferogram until the data is no longer indicative of the chemicalspresent. According to one embodiment, the processing subsystem (210;FIG. 2) may limit the effect of noise (440) by excluding noisyinterferogram regions from analysis. The processing subsystem (210; FIG.2) may apply the interferogram partitioning principle (IPP) to separatean interferogram into various sections to reduce noise, highlightcritical interferogram features, and increase analysis speed.

IPP teaches that individual fringes and sections of an interferogram maybe analyzed independently. The ability to partition and selectivelyanalyze interferogram fringes is a significant advantage over spectraldomain analysis. If only a portion of an interferogram is transformed tothe spectral domain, the resulting spectral plot usually containssignificant artifacts and distortions. Generally, spectral approachesmust transform an interferogram as a whole, including all noise in theinterferogram. However, the CIS system (110; FIG. 2) may use IPP toexclude noisy sections of an interferogram without any disadvantages inthe analysis. According to one embodiment, a threshold fringe index maybe determined so that interferogram fringes beyond the threshold areexcluded from analysis, eliminating the noise from interferogram basedanalysis.

The fringes used for chemical identification in the interferogram domainmay be selected from any section or sections of an interferogram. Forexample, one CIS embodiment may track noise levels at various fringescounting away from a center fringe (410; FIG. 4). The CIS system (110;FIG. 2) may determine that an interferogram (400) contains useful dataup to fringe 20 and also at fringes 33, 38 and 45. Other fringes may beignored to reduce the noise introduced into the analysis. The selectedfringes may then be compared to corresponding fringes of interferogramsin an interferogram database (220).

Many relationships between fringes and sections of an interferogram mayindicate the identity of compounds detected and other measurementproperties. According to one embodiment, interferograms may bepartitioned and compared according to fringe count, fringe placement,intensity, signal to noise ratio, shape, width, area, and othermeasures. For example, partitions according to fringe index and fringeposition may be used to characterize noise and determine backgroundlevels as described above. Additionally, partitions determined by fringeintensity may be compared to identify specific chemical signatures in aninterferogram (400).

Partitioning of interferograms using IPP may increase the accuracy ofchemical identification. According to one embodiment, the processingsubsystem (210; FIG. 2) may partition an interferogram according to thefeatures and fringes most likely to contain information regarding achemical species of interest. By comparing the most relevant sections ofa received interferogram and reference interferograms, the CIS system(110; FIG. 2) may increase sensitivity to specific chemicals andincrease analysis speed.

Multiple chemical signatures may be extracted from an interferogram toidentify the components and relative composition of mixtures of variouschemical compounds. When multiple chemicals are present, thecontribution of each chemical is proportional to the partial pressure ofeach chemical in the target region. The relative intensity of eachchemical signature in an interferogram may be determined by comparisonto reference interferograms in the interferogram database (220; FIG. 2).According to one embodiment, the relative intensity of each chemicalsignature may indicate the relative quantity of that chemical in thetarget region. According to one embodiment, quantitative results arepresented as percent composition of various chemicals present.Individual fringes, groups or ranges of fringes, entire interferograms,ratio interferograms, and other interferogram characteristics may beused to determine the appropriate weighting of chemical signatures todetermine the relative composition of a mixture of chemicals.

Interferogram data (400) may represent the composite of multiple sensorreadings integrated over a period of time and/or over multiple targetareas. Integration of multiple measurements allows small, consistentsignals to accumulate and emerge from noise. The integration of manyreadings reduces many types of noise while accentuating valid data. Thiscan improve the signal to noise ratio of the resulting interferogram.According to one embodiment, the intensities of multiple interferogramsare integrated at each corresponding data point. The result of theintegrations is a composite interferogram reflecting information fromeach individual interferogram. According to another embodiment, acomposite interferogram may also be scaled or normalized afterintegration. Additional embodiments may generate compositeinterferograms using processes including, but not limited to, summation,integration, piecewise construction, ratio of interferograms, andadditional transforms and mathematical operations.

To compensate for changing background conditions and varying signalintensities, interferogram data may be normalized before integration orcomparison to reference interferograms. According to one embodiment,interferogram data may be scaled so that the center fringe (410; FIG. 4)of each interferogram has equal intensity. Since the center fringeincludes contributions from all frequencies of IR waves received, theintensity of the center fringe (410; FIG. 4) closely approximates thetotal amount of IR waves received. Unfortunately, interferograms withlarge variation in maximum intensities cannot be easily compared.Scaling or normalizing the interferograms to have a common maximumintensity enables more accurate comparisons.

A background level or threshold may be determined to accurately detectchemicals introduced into a target area. FIGS. 6A through 6E illustratevarious exemplary embodiments of interferograms used to determinebackground intensity levels.

FIG. 6A illustrates an exemplary interferogram (400) representing ameasurement of ambient background noise. The interferogram (400) shownrepresents a measurement generated while no chemicals of interest arepresent in an appreciable quantity or concentration. The interferogram(400) may represent a single interferogram measurement or may representa composite interferogram, such as the integration of multiplemeasurements over a period of time and/or over multiple target areas.

FIG. 6B illustrates the interferogram (400) of FIG. 6A with a secondinterferogram (401). Both interferograms (400, 401) represent backgroundnoise measurements, but have somewhat different fringe placement andintensities. Since noise levels and background conditions oftenfluctuate, the comparison of multiple interferograms and measurementscan result in a more accurate background determination than a singlemeasurement. As illustrated, the two interferograms (400, 401) arescaled so that the central fringes have equal intensity. According toone embodiment, each interferogram (400, 401) may be a compositeinterferogram. For example, each interferogram (400, 401) may representthe integration of 10 background measurements.

The two background interferograms (400, 401) selected represent noiselevels at successive periods of time. For example, the overall noiselevel may be determined using a composite interferogram representing themost recent 10 measurements and another composite interferogramrepresenting the 10 earlier measurements. The comparison of backgroundinterferograms from multiple time periods can enable the CIS system(110; FIG. 2) to determine accurate noise levels when backgroundconditions are changing. According to another embodiment, the backgroundinterferograms may represent conditions at multiple spatial locations,measured during the same time period or different time periods. Forexample, analyzing background interferograms measured at adjacentsections of the target region may minimize background levelinconsistencies across the target region. Such comparisons may result ina “sliding window” of spatial and/or temporal comparisons to generate anaccurate dynamic background level.

As illustrated in FIG. 6C, both interferograms (400, 401) of FIG. 6Bhave been partitioned into multiple regions, indicated by verticaldividers (610). According to one embodiment, the partitions may besymmetrical about the central fringes of the interferograms (400, 401),as shown. However, since noise may be inherently asymmetrical,partitions are not required to be symmetrically placed. Partition sizesand locations may be determined based on a number of factors, including,but not limited to, properties of chemicals of interest, interferogramintensity levels, fringe count, fringe location, and fringe area.

According to the embodiment of FIG. 6C, a maximum and minimum intensitywithin each partition is determined, as illustrated by horizontalmarkers (600). According to one embodiment, these local maximum andminimum values may be used to set a background noise level within thatpartition. FIG. 6D illustrates noise ranges (620) derived from the localmaximum and minimum values from FIG. 6C.

According to another embodiment, the background level may be set as theabsolute value of the noise interferogram intensity within eachpartition. According to yet another embodiment, the background noiselevel may include an additional margin, such as the maximum intensityplus the difference between the absolute value of two interferogramswithin a partition. A variety of additional methods may be employed todetermine accurate background noise levels (620), and the method ofdetermination may vary according to background conditions and theconfiguration of the CIS system (110; FIG. 2).

According to one embodiment, the background levels (620) of FIG. 6D maybe used to detect significant changes in chemical content in the targetregion (120; FIG. 1) of the CIS system (110; FIG. 2). FIG. 6Eillustrates a comparison between a measured interferogram (400) andbackground noise levels (620). According to the embodiment of FIG. 6E,the interferogram (400) has been scaled for accurate comparison to thebackground levels (620). As shown, the interferogram (400) falls outsidethe noise levels (620) in certain partitions. According to oneembodiment, fringes (630) extending outside the noise levels (620) mayindicate the presence of a previously absent chemical in the measurementregion. According to another embodiment, interferogram (400) mayindicate the presence of a new chemical only when the interferogramfringes (630) exceed the noise levels by a predetermined percentage orthreshold. The processing subsystem (210; FIG. 2) may consider thedifference between the interferogram intensity and the background levels(620), the area outside the background levels (620), and otherinterferogram (400) properties to determine that an interferogram (400)represents the addition of a new chemical into the target region (120;FIG. 1).

According to another embodiment, chemicals in the target region (120;FIG. 1) may be detected by comparing measured interferograms (400) to acomposite background interferogram (400). For example, a number ofbackground interferograms may be integrated and scaled to representaverage background levels. The composite background interferogram may beupdated to reflect the most recent background measurements, such as thelast 100 background measurements. New interferogram (400) measurementsmay be compared to the background composite interferogram using a ratioof the new measurement with respect to the background. By taking theratio, noise in the resulting ratio interferogram tends to scale to avalue of 1 while interferogram features due to actual chemicals ofinterest maintain greater amplitudes. According to another embodiment,multiple measurements may be accumulated so that the ratio of acomposite measurement interferogram and a composite backgroundinterferogram is calculated.

After chemicals of interest have been detected, they may be compared toreference interferograms in the interferogram database (220; FIG. 2).Received interferograms may be scaled, checked for symmetry, accumulatedto form a composite interferogram, and/or checked for symmetry and otherquality measures before comparison to known interferogram chemicalsignatures. Measurement interferograms may be further processed,including taking a ratio with a background interferogram, as describedabove. Processed interferograms are then compared to interferograms inthe interferogram database (220; FIG. 2). According to one embodiment,individual fringes of the reference and measurement interferograms maybe compared. According to another embodiment, multiple fringes andranges of fringes may be compared as a group to identify chemicalsdetected in the interferograms. When a measured interferogram and areference interferogram are found to contain substantially similarcharacteristics, the chemical associated with the referenceinterferogram is considered to be present in the target region.

As described above, multiple reference interferograms, each representinga single chemical, may be compared to a measurement interferogram todetermine the identity of multiple chemicals and relative quantity ofeach chemical. According to one embodiment, multiple interferogramsignatures may be weighted and combined as described above.

FIG. 7 illustrates the steps of an exemplary method for determining abackground noise level for a comparative interferogram spectrometry(CIS) system in ambient conditions, according to one embodiment. Themethod described may be applied to generate a background level for theentire target region or a portion of the target region, such as a pixelor group of pixels within a measured area.

In step 700, one or more interferograms describing background noise arereceived. According to one embodiment, interferograms may be receivedfrom an interferometer measuring incident IR waves from a target region.Interferograms may also be received through a network, local storage,removable media, and other sources.

Next, the quality of a received interferogram is determined (step 710).According to one embodiment, various interferogram characteristics maybe considered, including, but not limited to, interferogram symmetry,fringe placement, fringe shape, fringe area, and ratios of thesecharacteristics. Interferogram quality measures may be compared topredetermined or dynamic thresholds to determine if an interferogram isvalid. Analysis of an interferogram may also include adjusting receiveddata to compensate for instrument error and adjusting quality thresholdsfor measurement conditions. Interferograms that are found to be invalidare discarded unless corrections can be made, and valid interferogramsare retained for further analysis. A series of interferograms belowquality thresholds may indicate that the interferometer or othercomponent of the CIS system (110) has failed.

Valid background interferograms are then accumulated to create one ormore composite interferograms (step 720). As previously mentioned,interferograms may be scaled to appropriate intensity levels beforecomposite interferograms are formed. Additionally, interferogramsincluding chemicals signatures outside of background levels may beexcluded from accumulation or integration. Composite backgroundinterferograms may include data from measurements from multiple spatialsections of a target region and time periods, as mentioned previously.

Composite background interferograms are then used to set a backgroundnoise level (step 730). According to one embodiment, multiple compositebackground interferograms may be compared to determine appropriate noiselevels. One embodiment may compare the previous two compositeinterferograms generated, for example, to determine a dynamic backgroundbased on the most recent background measurements. Background levels maybe represented as a set of noise thresholds, a composite noiseinterferogram, of other representation. According to one embodiment, theexemplary method of FIG. 7 may repeat as frequently as desired,beginning at step 700, to determine an updated background level.

FIG. 8 is a flowchart illustrating a method of identifying chemicalsbased on interferogram signatures, according to one exemplaryembodiment.

In step 800, one or more measurement interferograms are received.According to one embodiment, interferograms may be received from aninterferometer measuring incident IR waves from a target region.Interferograms may also be received through a network, local storage,removable media, and other sources.

According to one embodiment, interferograms may be filtered or selectedprior to step 800 so that only valid measurements are received andprocessed using the method of FIG. 8. According to another embodiment,step 800 may include receiving valid and invalid interferograms, andstep 800 may additionally determine the quality of receivedinterferograms, as described in step 710 of FIG. 7. Interferograms thatare received and determined to be valid in step 800 may be furtherprocessed while invalid interferograms may be discarded.

Received interferograms are then compared to background noise levels(step 810). According to one embodiment, a ratio interferogram may begenerated by dividing received interferograms by a composite backgroundinterferogram. Noise contributions in the received interferogram arereduced to values close to one, while significant chemical signaturesremain at greater amplitudes. Signal intensities of the ratiointerferogram above a threshold may indicate the presence of chemicalsother than those in the background. A section of the target region, suchas a set of pixels with fringe intensity above a background level, maybe designated for further processing and identification.

According to another embodiment, multiple received interferograms areaccumulated and integrated to generate a composite interferogram, asmentioned above. The ratio interferogram is then generated from thecomposite measurement interferogram and the composite backgroundinterferogram. Using composite interferograms spanning multiplemeasurements, the signal to noise ratio can be significantly increased.Alternately, individual interferogram measurements may be processed insome embodiments.

Ratio interferogram and/or measurement interferograms containingchemical signatures are then compared to reference interferograms toidentify the chemicals indicated by the interferograms (step 820). Thefeatures of the ratio interferograms, measurement interferograms, andvarious reference interferograms may be compared in any of the waysdescribed above, including, but not limited to, by fringe intensity,placement, shape, and area. As mentioned previously, various partitionsof the interferograms may be compared individually. Sections of theratio and measurement interferograms with high noise levels may beexcluded from comparison according to the interferogram partitioningprinciple. Interferograms with features that correlate with those of areference interferogram are considered to indicate the presence of thechemical or chemicals used to generate the reference interferogram.

For example, one reference interferogram generated from a particularchemical may contain significant intensity at fringes 13 and 17, wherethe intensity at fringe 17 is approximately double the intensity atfringe 13. Ratio and/or measurement interferograms which contain theseor substantially similar characteristics may be considered to have beengenerated from the same chemical as the reference interferogram.

According to another embodiment, a user of the CIS system (110) may benotified during or subsequent to step 820 if no reference interferogrammatches the characteristics of the interferogram being processed.Additionally or alternately, the characteristics of the unmatchedinterferogram may be recorded to update the database of referenceinterferograms for future comparisons. According to one embodiment,measurement and ratio interferograms may be analyzed in step 830 todetermine the relative composition of the chemicals identified in step820. Generally, determination of the percentage composition of chemicalsidentified is performed when at least two chemicals are identified instep 820. Quantitative analysis, including percentage compositioncalculations, may be performed in any of the ways described above,including, but not limited to, analysis by partial pressures, comparisonof intensity ratios of individual chemical signatures, and comparison tomultiple reference interferograms.

The preceding description has been presented only to illustrate anddescribe various embodiments of the principles described herein. It isnot intended to be exhaustive or to limit the principles describedherein to any precise form disclosed. Many modifications and variationsare possible in light of the above teaching.

1. A method for determining a background noise level, said methodcomprising: receiving interferogram data; determining at least onemeasure of interferogram quality; generating a background noise levelbased on said interferogram data and at least one measure ofinterferogram quality.
 2. The method of claim 1, further comprisingaccumulating said received interferogram data.
 3. The method of claim 2,wherein said accumulating step includes generating at least onecomposite interferogram.
 4. The method of claim 2, wherein saidaccumulating step excludes said received interferogram data determinedto have a quality less than a threshold quality.
 5. The method of claim1, wherein said background noise level is represented as a set ofthreshold interferogram intensity values.
 6. The method of claim 1,wherein said background noise level is represented as a compositeinterferogram.
 7. The method of claim 1, wherein said generating stepincludes comparing received interferograms from at least two timeperiods.
 8. The method of claim 1, wherein said generating step includescomparing received interferograms from at least two spatial locations.9. The method of claim 1, wherein said method is repeated to determine adynamic background noise level.
 10. The method of claim 1, furthercomprising receiving sample interferogram data from a material understudy; comparing said sample interferogram data to said background noiselevel; and comparing said sample interferogram data to at least onereference interferogram to identify at least one chemical indicated bysaid sample interferogram data.
 11. The method of claim 10, wherein saidbackground noise level comprises a composite interferogram.
 12. Themethod of claim 10, wherein said comparing to a background noise levelstep comprises generating a ratio interferogram between said receivedinterferogram data and said background noise level.
 13. The method ofclaim 12, wherein said ratio interferogram is compared to said at leastone reference interferogram.
 14. The method of claim 10, furthercomprising determining at least one quality measure of said sampleinterferogram data.
 15. The method of claim 14, further comprisingexcluding sample interferogram data determined to have a quality measureless than a threshold quality measure from further comparisons.
 16. Themethod of claim 10, further comprising determining a percent compositionof chemicals identified in said comparing to at least one referenceinterferogram.
 17. The method of claim 10, wherein said comparing to abackground noise level step comprises determining if an intensity of atleast one fringe of said received interferogram data exceeds abackground intensity measure of said background noise level.
 18. Themethod of claim 10, wherein said comparing to at least one referenceinterferogram step comprises comparing selected partitions of saidreceived interferogram data and said at least one referenceinterferogram.
 19. The method of claim 18, wherein said partitions areselected according to at least one of fringe position, fringe intensity,fringe shape, fringe intensity, fringe area, and background noise level.20. The method of claim 19, wherein said partitions exclude at least aportion of noise from said interferogram data.